This invention relates to a direct iron making technology which utilizes a shaft furnace or a fluidized bed furnace, more particularly, to a method for producing iron carbide (Fe.sub.x C.sub.y, x/y=2 to 3) from iron ore efficiently.
Iron carbide has been attracting considerable attention in recent years as a new, high-quality iron source material capable of realizing a new generation of energy-saving iron making technology which produces a smaller amount of carbon dioxide gas. There have been known some methods for producing iron carbide. These include a Stelling method disclosed in U.S. Pat. No. 2,780,537 in which fine iron ore is brought into contact with gas containing carbon monoxide (CO) at a temperature between 400.degree. C. and 900.degree. C., and an Iron Carbide method disclosed in U.S. Pat. No. 4,053,301 in which fine iron ore is reduced by H.sub.2 gas at a temperature between 595.degree. C. and 705.degree. C. in a fluidized bed and carburized by a carbon-containing substance at the same time.
In the Iron Carbide method, when the treatment temperature is raised to improve the reduction efficiency, metallic iron produced in a furnace is liable to cause the sticking phenomenon derived from the surface characteristics of the produced metallic iron, making it difficult to ensure the stable operation. In producing sponge iron or iron carbide, there is liable to occur an undesirable phenomenon that pipes are plugged by free carbon or soot caused by precipitation from CO and CH.sub.4 in a reducing gas. Another undesirable phenomenon is brittle fracture of a reactor vessel, for example, due to carburizing reaction. To overcome these problems, the operation temperature is kept in a lower range or the concentration of carburizing gases such as CO and CH.sub.4 is controlled to be slightly higher than that at which Fe/Fe.sub.3 C equilibrium is attained.
Japanese Examined Patent Publication No. 44-14462 discloses a method for suppressing conversion of CO to free carbon or to carbon reactable with iron in the process of sponge iron production. In this method, specifically, 1 to 1000 ppm by volume of sulfur compounds is added to the reducing gas within a temperature range of 426.degree. C. to 816.degree. C.
However, the productivity of the conventional iron carbide producing methods is extremely low. This is because iron carbide is produced at relatively low treatment temperatures and with carburizing gas composition close to the Fe/Fe.sub.3 C equilibrium, that is, under conditions undesirable for reducing and carburizing reactions.
In view of these problems, the inventors of this application have studied to improve the productivity of iron carbide, and discovered that it is possible to 1) suppress precipitation of free carbon without hindering iron carbide production; and 2) prevent decomposition of the produced iron carbide by adjusting the activity of sulfur contained in a reducing gas containing hydrogen and a carbon compound and by causing the iron carbide to adsorb sulfur on its surface. Based on this discovery, the inventors succeeded in producing iron carbide at a remarkable high efficiency.
However, there has been yet a problem to be solved. Specifically, the reducing gas usually contains oxidizing gas, such as H.sub.2 O, CO.sub.2. It has been noticed that the oxidizing gas hinders the production of iron carbide, and give adverse influence to the production of iron carbide. Also, it is usually impossible to remove the oxidizing gas from the reducing gas entirely. This requires fine and delicate control of composition of reducing gas in accordance with various reaction conditions, thus impairing the industrial applicability.